Slide a belt sleeve onto the belt.
Route the belt down through the slot in the belt clip and then wrap the belt back under itself. The teeth from the short end of the belt should engage with the teeth on the long end.
Slide the belt sleeve over both ends of the belt. This may take a small bit of force and some wiggling.
Mount the clip to the end of an extrusion with a 20mm nut bar and M5 x 10mm screws.